Vehicles such as trucks, buses, and construction machines utilize compressed air sent from a compressor, which is directly connected to an internal combustion engine (hereinafter, referred to as an engine) to control systems such as brakes and suspensions. The compressed air contains water, which is contained in the atmosphere, and oil for lubricating the inside of the compressor. When the compressed air containing such water and oil enters inside the systems, it causes rust and swelling of rubber members (such as O-rings) and results in an operational defect. Thus, an air dryer is provided downstream of the compressor in a pneumatic system for removing water and oil from the compressed air (for example, Patent Document 1).
A filter and a desiccant such as silica gel and zeolite are provided in the air dryer. The air dryer performs dehumidification to remove water from compressed air and regeneration to regenerate the desiccant by removing the water adsorbed by the desiccant and discharging it to the outside.